Steel is an alloy that is made up of iron with less than 2% carbon and is most often mixed with a small amount of other alloying agents such as silicon, manganese, sulfur and phosphorus. Generally speaking steel has good mechanical properties and is workable through plastic deformation and machining and can be easily welded.
The Steel Making Processes
The mechanical properties of the different types of steel depend on the amount of carbon added, which determines its hardness, on the other alloying elements added and the thermal treatment it undergoes.
Adding more carbon to the alloy mix will:
- increase its ultimate tensile strength (UTS), hardness, hardenability, castability and wear resistance
- decrease its elongation and consequently its ductibility, cold workability and formability, and its welding ability
Based on the amount of carbon in the chemical composition, steel can be categorized as:
- low carbon steel which contains less than 0.2% of carbon
- medium carbon steel which contains between 0.2-0.77% of carbon
- plow steel which contains more than 0.77% of carbon
The presence or otherwise of alloying agents further characterizes the steel as:
- non-alloy or carbon steel if the steel does not contain any additional alloying agents
- low alloy steel when all alloying agents are below 5% each
- alloy steel when at least one of the alloying agents is more than 5% of the mix
Steel foundry production processes:
- Continuous or Strand Casting: This is a process whereby the metal is poured into an open based water cooled mold. The metal solidifies and is extracted from the other side of the mold, and the process is continuous
- Casting Ingots: Ingot casting is less commonly used but it is the only known method to produce some low-alloy steel grades and steel grades for special uses. Using this process, the metal is bottom-poured into a stationary mold for the smaller ingots and vacuum stream degassing is used to pour the molten steel into the larger ingot molds
- Sand mold casting: The sand molds used must have a high degree of refractoriness to ensure that they do not disintegrate under the high temperature of the molten steel. Sand mold casting facilitates the production of geometrically intricate parts that only require rough machining and a mill finish once they are cast (see also Aluminum Sand Casting). There are several different types of sand mold casting used to cast steel products such as:
- Green Sand Casting where the cast is made out of clay mixed with substances such as silica, olivine, chromite, and bonded with betonite and mechanically pressed
- No-bake or cold box casting where the sand is bonded with organic and inorganic chemical compounds and hardened at room temperature
- Shell mold casting using a similar casting method to sand casting with the difference that the mold is shaped by applying a sand-resin mixture around a metal pattern to form a thin shell around it
- Investment casting also known as lost wax process makes use of a wax pattern around which a ceramic mold is built which is then used for the casting. Like ceramic mold casting investment casting produces a net shape part and requires little machining
- Centrifugal casting which uses a metal die that is rotating
There are two types of furnaces used in steel casting:
- Electric induction furnace. This furnace makes use of eddy currents to raise the temperature of the metals until it liquefies
- Electric-arc furnace: This furnace uses the electric arc generated by graphite electrodes to melt the metal
Following its production the steel or steel parts are subjected to further thermal or chemical treatment to improve their thermo-mechanical properties. Some of the possible treatments are:
- Quenching, a treatment intended to harden the steel
- Tempering, intended to increase ductility and toughness but at the cost of a lower degree of hardness and resistance
- Annealing which softens the steel and renders it more malleable
- Normalization which is used to eliminate the internal stresses induced by machining and to homogenize the structure of the castings
- Carburizing which is a treatment that is used on steel with low carbon content. It exposes it to a carbon rich material at a temperature of about 900°C which is below the steel’s melting point; this releases the carbon from the carbon-rich material which is absorbed by the steel surface. When the steel is subsequently put through a surface quenching process, a martensite case forms over the low-carbon steel workpiece
- Carbonitruration is a similar treatment to carburizing but also includes atomic nitrogen in the treatment which is carried out at a temperature of approximately 800°C and is always followed by surface quenching
- Nitriding is a complete treatment that does not require quenching. It diffuses nitrogen to produce a hardened case by forming surface nitrides (1200HV)
Virtual Engineering the Steel Casting Process
A comprehensive simulation of steel production processes for both continuous and ingot casting as well as for the foundry casting of steel workpieces can be carried out starting with the filling phase and ending with the solidification phase. For the latter a detailed simulation study to predict the solute segregation is important. Segregation is classified as either micro-segregation or macro-segregation depending on its scale. Micro-segregation is the result of solute redistribution during solidification and macro-segregation, results from the fluid flow and solutal convection. An accurate prediction of the level of macro-segregation expected from a given process depends largely on the accurate prediction of the micro-segregation that takes place.
For foundry steel casting the following additional simulations can be carried out:
- a virtual simulation of the heat treatment to analyze the expected mechanical properties and microstructure of the finished part
- a stress-strain analysis of the newly cast part and the cast part after heat treatment to predict the stress condition and plastic deformation of the workpiece and to identify any areas where cracks such as hot tears could develop
The casting simulation should include a full fluid dynamic study. A few of the things it should consider are:
- the effects of turbulence that can foster the formation of oxides and inclusions
- high velocity values for the beginning of the flow which can result in the formation of foam and can cause the sand cast to be eroded changing its shape
- vortices that can cause air entrapment
- sudden drops in the temperature of the front flow which can cause cold shuts
Following a complete fluid dynamic study the solidification and cooling phases should be simulated to analyze:
- the presence of shrinkage porosity
- overheating of the sand molds which could cause surface inclusions in the cast part; cracks in the sand mold that would cause shape distortion of the workpiece cast
- the effectiveness of chills
- dimensional quality of the workpiece
Using the segregation and convection micro models such as those found in MAGMASteel (the steel casting module of MAGMASoft), it is further possible to verify the concentration of the various elements of the alloy at various points in the casting. MAGMASteel's heat tratment module also makes it possible to verify the microstructural characteristics and the mechanical properties of the steel at end of each state change.
Finally by applying the stress analysis to the cast part before or after heat treatment, it is possible to predict problems resulting from hot cracking and the evolution of stress and deformation from the solidification phase right through its reaching ambient temperature to evaluate the pretension of the casting and its shape within the expected tolerances.
The Advantages of using Virtual Simulation for Steel Casting Projects
The virtual simulation of a steel cast object allows designers to improve product geometry without the need of casting several physical prototypes, and to make changes to the entire production process in order to produce a better quality workpiece more efficiently.
An integrated virtual simulation study starting with the design phase through the finished product allows the results of the structural analysis study to be passed on to the FEM code for a structural assessment of the workpiece. This integrated analysis allows the engineer to optimize both the part design and process variables used throughout the manufacturing process in order to attain a higher quality part, while substantially reducing its overall design and production costs. Reduced costs will come from a reduced quantity of raw materials needed, from reduced labor to produce it (thanks to the improved process automation and fewer physical prototypes needed), from a reduced time-to-market and from reduced metal waste.
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